The present invention relates generally to providing an estimation of a fuel cell stack inlet and outlet humidity levels, and more particularly to devices and methods for determining relative humidity during fuel cell operational transients without requiring relative humidity feedback from a sensor.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode to facilitate the electrochemical conversion of hydrogen and oxygen into electrons and positively charged ions (for the hydrogen) and negatively charge ions (for the oxygen). The electrons flow through an external electrically-conductive circuit (such as a load) to perform useful work, and then on to the cathode. An electrolyte layer separates the anode from the cathode to allow the selective passage of ions to pass from the anode to the cathode. The combination of the positively and negatively charged ions at the cathode results in the production of non-polluting water as a by product of the reaction. In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, the electrolyte layer is in the form of a proton-transmissive membrane; the layered structure formed by this PEM sandwiched between the anode and cathode is commonly referred to as a PEM electrode assembly (MEA). Each MEA forms a single fuel cell, and many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output. The PEM fuel cell has shown particular promise for vehicular and related mobile applications.
Balanced moisture or humidity levels are required in the PEM fuel cell to ensure proper operation and durability. For example, it is important to avoid having too much water in the fuel cell, which can result in the blockage of reactants to the porous anode and cathode. Contrarily, too little hydration limits electrical conductivity of the membrane, and in extreme cases can lead to it wearing out prematurely. As such, it is beneficial to have knowledge of the hydration level within a fuel cell, especially PEM fuel cells that frequently operate at elevated temperatures that can impact a cell's hydration level.
High frequency resistance (HFR) is a known diagnostic technique for indirectly measuring MEA hydration. In a typical HFR configuration, sensors use a high-frequency ripple current to measure fuel cell resistance. Although such an approach is particularly sensitive to changes in relative humidity (RH), its sensitivity to other fuel cell conditions can cause erroneous measurements. In other words, the measured fuel cell resistance, or HFR value, is measuring the resistance of the build-up of water in the PEM of the fuel cell and not the wetness of the air. The air has to dry or dampen the PEM for a change to occur in the HFR value. One particular weakness of HFR-based estimation is the inherent lag in HFR, especially at low flow conditions which exhibits a hysteresis response in the HFR value. This hysteresis response means that in situations where rapid inlet humidity changes are present, such changes will not match an average stack HFR value that often lags. This lag may cause a controller to over-dry the stack; such over-drying is particularly prevalent at the cathode inlet, where chemical degradation and consequent PEM thinning may ensue. As such, it remains challenging and difficult to provide accurate estimations of relative humidity levels in a fuel cell system. This is particularly acute in vehicular-based fuel cell systems where reliability, weight and cost further compound the challenges. In conventional configurations, to accomplish monitoring the RH of the stack, a separate humidity sensor is used to measure the RH of the air flowing through the stack. The humidity sensor allows a control system to determine the humidity of the PEM without succumbing to the lag or hysteresis response of the HFR sensors in response to operational transients. Unfortunately, such humidity sensors add cost and complexity to the system.
A cathode humidification unit (CHU) model algorithm is used to interpret and modify the RH of the stack. Variations in the effectiveness of the CHU model may be due to part-to-part variation, degradation, or even leaks. Degradation may depend on usage profile and may be different from vehicle-to-vehicle. Furthermore, the CHU model uses an outlet RH value of the stack for the calculations of the RH of the stack. If the outlet RH value has an error, (e.g. due to error in stoichiometry estimation, temperature feedback or even variation in anode water crossover) this would impact the CHU model's ability to calculate the RH of the stack and cause a circular reference.
A reference signal would benefit the CHU model to calculate the RH of the stack without the need for the humidity sensor.